1. Field of the Invention
This invention relates to a prosthesis which is surgically implantable into a body joint, such as the knee.
2. Background Art
Current knee arthroplasties typically involve replacement of the arthritic joint surfaces and are known for their use of metal and plastic components. They are usually embedded in a polymethylmethacrylate (PMMA) cement mantle to adhesively and mechanically bond the components to the area of bone exposed during the course of surgery.
Typically, this area of exposed bone is 2-3 mm below the area of existing eroded bone surface and generally requires removal of the entire subchondral bone in the area of implant location. In all cases, the subchondral (SC) bone of the tibial plateau, which is attached to the remaining articular surface, is removed as standard practice for both the total knee (TKR) and partial or unicompartmental knee (UKR) replacement procedures.
Further, FDA guidelines generally dictate that when polyethylene (PE) is used as a bearing surface, whether in conjunction with a metal support plate or not, at least 6 mm of PE thickness must be used to prevent fracturing of the PE during use. When the PE is used on the tibial side of these implant designs, this requirement leads to bone resections of the tibial plateau generally greater than 7 mm. The subchondral bone thickness on a typical tibial plateau is generally 2-3 mm. Thus, a typical TKR or UKR implant will require resection of the entire SC bone present on the tibial plateau, leaving only cancellous bone.
The PE is typically held in place by an interference fit or by melt infusion to a metal backing plate known as the tibial baseplate. This baseplate, in turn, is held in place on the now exposed cancellous bone of the tibia by screws, keels, posts, or combinations of some or all of these devices. The screws and keels generally provide immediate fixation, but these are usually enhanced by the addition of the PMMA cement. In the case of perforated keels, tapered and hourglass shaped posts, when these projections are set in uncured cement, the cement forms around and through them and, once hardened, provides an almost indestructible bond between the PMMA cement and the tibial baseplate. The cement also permeates the open cellular structure of the cancellous bone, thus resulting in the same type of bond between the bone and the tibial baseplate. In some unique cases, the metal tibial baseplate is not used and an all-PE design is bonded directly to the cancellous bone with the PMMA cement utilizing a roughened PE surface or molded posts to facilitate the bond with the PMMA cement.
A new generation of tibial hemiarthroplasty (THA) implant designs has been introduced which do not require significant resection of the SC bone of the tibial plateau to function properly. Examples of this are the U.S. Pat. Nos. 6,206,927; 6,558,421; 6,966,928; 6,866,684; and 7,341,602, each of which is incorporated by reference herein. These THA designs maintain their proper location in the joint by interference with preexisting or prepared anatomical shapes present in the knee joint, and none require PMMA-cemented protrusions or screws for proper function.
Previous keel designs, whether utilized for THA, UKR, or TKR implants, typically utilize an anteriorly-oriented keel. For example, the Zimmer Sbarbaro “skate” implant has a keel aligned in the anterior-posterior (AP) direction, with the posterior portion being rounded and sharpened and the anterior portion having an anterior (forward) pointing distal tip. In order to insert this particular shaped keel into a tibia with a cut to accept the keel, the length of the saw cut needed to insert the bottommost portion of the keel would be significantly longer the length of the keel at the base of the implant, thus allowing the implant to be able to slide in an anterior fashion upon implantation in an anterior to posterior insertion direction.
Other previous keel designs, such as the DePuy “Preservation” UKR, utilize a keel which extends the majority of the length of the baseplate. In this implant, the keel also comprises an hourglass shape in medial-lateral (ML) section. Due to the length of the keel and the hourglass design along its length, this implant cannot be inserted into the tibia without first making a substantial femoral cut to provide access to the tibia, or otherwise inserting the implant via a lengthwise insertion from the most anterior portion of the tibial plateau. In other words, the tibial plateau, rather than receiving a simple angle saw cut in order to receive the implant, must have a milled hourglass shape cut in the plateau which extends through the most anterior cortical bone in order for the keel to be inserted into the joint. If such a milled cut is not prepared, then a cut equal to the largest width of the keel must be made, which would not provide positive locking with the keel unless a mantle of cement is used.